To date, there is an absence of plasmid DNA vectors that are considered safe, potent, and efficient for medicinal use. The presence of antibiotic resistance genes in the delivered plasmids is one of the drawbacks of modern gene therapy and DNA vaccine applications.
Plasmids are extra-chromosomal DNA molecules that are separate from the chromosomal DNA and are capable of replicating independently of the chromosomal DNA (Lipps G. (editor). (2008). Plasmids: Current Research and Future Trends. Caister Academic Press. ISBN 978-1-904455-35-6). Plasmids usually occur naturally in bacteria, but are sometimes found in eukaryotic organisms. They are considered transferable genetic elements, or replicons, capable of autonomous replication within a suitable host. Plasmids are naked DNA and do not encode genes necessary to encase the genetic material for transfer to a new host. Therefore host-to-host transfer of plasmids requires direct, mechanical transfer by conjugation or changes in host gene expression allowing the intentional uptake of the genetic element by transformation (Lipps G. 2008).
The use of plasmid DNA (pDNA) for gene therapy and vaccination is a novel technology with considerable potential in human and animal health care (Mairhofer, J. et al., Biotechnol. J., 3, 83-89, 2008). In addition, plasmids serve as important tools in genetics and biotechnology labs, where they are commonly used to multiply or express particular genes (Russell, David W.; Sambrook, Joseph (2001), Molecular cloning: a laboratory manual. Cold Spring Harbor, N.Y: Cold Spring Harbor Laboratory).
Plasmids provide a mechanism for horizontal gene transfer within a population of microbes and typically provide a selective advantage under a given environmental state. For example, plasmids may carry genes that provide resistance to naturally occurring antibiotics. Alternative markers besides antibiotics also exist. For example, the proteins produced by the plasmid may act as toxins which also provide a selective advantage under a given environmental state. Plasmids also can provide bacteria with an ability to fix elemental nitrogen or to degrade recalcitrant organic compounds which provide an advantage under conditions of nutrient deprivation (Lipps G., 2008).
For selection using antibiotic resistant genes, plasmids are prepared wherein a gene is inserted that generates a protein which makes cells resistant to a particular antibiotic. Next, the plasmids are inserted into bacteria by a process called transformation. Then, the bacteria are exposed to the particular antibiotics. Only bacteria which take up copies of the plasmid survive the antibiotic, since the plasmid makes them resistant.
Genes of interest may be delivered using a plasmid containing an antibiotic-resistant marker. Such a gene is typically inserted at a multiple cloning site (MCS, or polylinker). The antibiotic-resistant genes are expressed and the expressed proteins break down the antibiotics. In this way the antibiotics act as a filter to select only the modified bacteria. Now these bacteria can be grown in large amounts, harvested and lysed (often/using the alkaline lysis method) to isolate the plasmid of interest.
The very success of antibiotics in medicine has now become a problem. Many bacteria, including pathogens of infectious diseases, are already resistant and can no longer be controlled with the particular antibiotic. Antibiotics have been used too frequently in human and animal medicine. Moreover, of even far greater significance is the fact that for a long time they were added to animal feed as a performance enhancer. This practice is now largely outlawed, but the pervasive antibiotics have given a survival advantage to those bacteria that have a corresponding resistance gene. Moreover, resistance genes in bacteria are often located on mobile DNA units, which can be exchanged between different species.
Against this background there is concern that bacteria could absorb marker genes eventually resulting in pathogens, against which antibiotics currently being prescribed are ineffective. There might be a gene transfer to environmental microorganisms, e.g., pathogens (Murphy, D. B., Epstein, S. L., Guidance for Industry: Guidance for human somatic cell therapy and gene therapy, Food and Drug Administration, Rockville 1998). Another safety concern is the possible integration of the antibiotic resistance gene into the human chromosome (Smith, H. A., Klinman, D. M., Curr. Opin. Biotechnol. 12, 299-203, 2001). Further, such genes may have a considerable impact on the plasmid production process, as constitutive expression of these genes imposes an unnecessary metabolic load on the bacterial host cell (Cranenburgh, R. M., et al., Nucleic Acids Res. 2001, 29, e26; Rozkov, A., et al., Enzyme Microb. Technol. 2006, 39, 47-50). Reducing the size of the plasmids by eliminating these genes would lead to improved stability and yield of pDNA obtained by the fermentation process (Smith, M. A., et al., Can. J. Microbiol. 1998, 44, 351-355).
Therefore, there is an absolute need in the art to avoid the use of antibiotic resistance genes in the final (commercial) product (naked DNA vaccine) as there is a potential acceptance risk by the public/consumers following current recommendations from regulatory authorities. The Food and Drug Administration (FDA) and the World Health Organization (WHO) regulate the use of antibiotic resistance markers to assure the quality of DNA vaccines and to prevent infectious diseases. Similarly, the EU Deliberate Release Directive, which has been in effect since 2002, requires “the phasing out of the use of antibiotic-resistance markers in genetically modified organisms which may have a harmful impact on human health or the environment”.
The drawbacks of traditional markers are becoming apparent even in practical research. For example, there is a need to have an antibiotic-free delivery system for the commercial applications of bactofection technology. Bactofection technology is the delivery of plasmid DNA into eukaryotic cells using invasive bacteria. Moreover, there exists a technical need to reduce unnecessary metabolic burdens during the fermentation process, which will achieve higher ODs and higher yields in DNA plasmid.
Alternative selection strategies have been designed to address concerns regarding dissemination of antibiotic resistance genes to a patient's enteric bacteria including auxotrophy complementation, repressor titration, protein based antidote/poison selection schemes, and the use of RNA based selectable markers (see Williams J. A. et al., Plasmid DNA vaccine vector design: Impact on efficacy, safety and upstream production, Biotechnol Adv (2009), doi:10.1016/j.biotechadv.1009.02.003).
Cranenburgh, R. M. et al. reported the construction of two novel Escherichia coli strains (DH1lacdapD and DH1lacP2dapD) that facilitate the antibiotic-free selection and stable maintenance of recombinant plasmids in complex media. They contain the essential chromosomal gene, dapD, under the control of the lac operator/promoter (Cranenburgh, R. M., et al., 2001). Unless supplemented with IPTG (which induces expression of dapD) or DAP, these cells lyse, however, when the strains are transformed with a multicopy plasmid containing the lac operator, the operator competitively titrates the Lad repressor and allows expression of dapD from the lac promoter. Thus transformants can be isolated and propagated simply by their ability to grow on any medium by repressor titration selection. No antibiotic resistance genes or other protein expressing sequences are required on the plasmid, and antibiotics are not necessary for plasmid selection.
Mairhofer et al. recently investigated designing bacterial host strains that serve to select and maintain plasmids without the use of any selection marker or other additional sequence on the plasmid. Several bacterial strains were modified, so that the plasmid's replicational inhibitor RNA I could suppress the translation of a growth essential gene by RNA-RNA antisense reaction (Mairhofer, J. et al., Biotechnol. J., 3, 83-89, 2008). An essential gene (murA) was modified such that a repressor protein (tetR) would hamper its expression (Mairhofer, J. et al., 2008). Only in the presence of plasmid and, hence, RNA I, was tetR turned down and murA expressed. The authors reported that different commercially available plasmids could be selected by various modified Escherichia coli strains. They further designed a minimalistic plasmid devoid of any selection marker.
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